In my prior U.S. Pat. No. 2,774,239 issued Dec. 18, 1956 there is disclosed an apparatus for determining the dynamic mechanical properties of viscoelastic materials. This previously patented apparatus and method has yielded the complex compliance and complex modulus of materials ranging from viscous liquids and soft gels, thermoplastics and elastomers, to hard solids such as quartz, lead, and stainless steel. Although this prior measurement method and apparatus have been successful, there are substantial and significant drawbacks and limitations. First the measurement requires two, separate alternating current electrical bridge balances. Further, the two coaxial coils used for excitation and for velocity sensing are tightly coupled electromagnetically through a soft iron pole piece, and a pre-balance must be made, before each bridge balance, in order to eliminate or minimize the (non-motional) mutual inductance voltage in the velocity sensing coil by means of auxiliary shielding and test coils. In addition, circular, coaxial coils are arranged to be in annular gaps between pole pieces whose reluctance decreases the magnetic flux density. Thus, a very large permanent magnet must be used to assure the necessary high value of magnetic flux density in the gap. This large magnet requirement and the iron pole pieces needed results in a large and massive apparatus which, in turn, prevents rapid temperature changes and slows the acquisition of information on temperature dependence of dynamic mechanical properties. Finally, the determination of the complex modulus at each frequency, because of the two necessary bridge balances and the prebalance to eliminate mutual inductance, requires 10 to 20 minutes.
The vibration excitation and sensing element in the prior apparatus consists of an aluminum alloy driving tube with coaxial coils as mentioned above, and it and the associated mechanical and electrical equipment are described in detail in U.S. Pat. No. 2,774,239.